1. Field of Invention
The present invention relates to a video amplifier circuit for cathode-ray tube display monitors which are used as output display terminals (computer peripherals) or for other display purposes.
2. Prior Art
In cathode-ray tube (abbreviated as CRT hereinafter) display monitors, when characters, symbols, or graphics are displayed, it is often required to accentuate some parts of the display. A dual intensity display mode, whereby the parts desired to be accentuated are displayed with a higher intensity level and the remaining parts not to be accentuated are displayed with a lower intensity level, is commonly used for this purpose.
In this dual intensity display mode, it is desirable that the intensity difference between the accentuated parts and the remaining parts on the display CRT is continuously adjustable with a control knob provided, for example, on a front panel of the display monitor apparatus. However, signals processed in digital circuits such as used in computers have normally only two discrete levels: High Level (called "H" hereinafter) and Low Level (called "L" hereinafter). Achieving the intensity difference adjustment mentioned above with signals of two discrete levels presents certain difficulties in designing circuits of such the display monitor apparatus and inconveniences described later in their operation.
In the following the configuration of a typical approach of the prior art and its drawbacks are explained. FIG. 1 shows an example of the display on a CRT screen in the dual intensity display mode. In this example, for simplicity, only characters are displayed. Therefore, hereinafter the word "character" is to be understood to represent symbol or graphic element also. The characters with boldface letters represents those characters which are accentuated with a higher display intensity level from the remaining non-accentuated characters with a lower display intensity level, which are represented with lightface letters.
Hereupon, in the relation to the later explanations, the principle of generating and displaying characters on the CRT screen is simply explained below. In the CRT display using raster scan TV mode, the individual characters are generated with appropriately located combination of dot matrix elements as shown in FIG. 2, wherein a capital letter A is shown as an example with the 5 by 7 dot matrix. Dot matrix elements of each same row are scanned by a single horizontal scan of an electron beam of the CRT. Those dot matrix elements which are to be in a "H" state are brightened. The whole character comprizes seven horizontal scanning lines "H1" to "H7". The dot matrix elements in the "H" state on these seven scanning lines, shown by the solid circles (black circles), form a capital letter A, and the rest of the matrix elements, in the "L" state, shown by the open circle (white circles), remain unbrightened.
Now, for explaining the operation of the dual intensity display mode, the character-generating video signal (also referred to herein as an "output" character-generating video signal, to be distinguished from the "input" character-generating video signal (see below)) and the dual intensity signal are shown in FIG. 3, wherein (a) shows the character-generating video signal in the dual intensity display mode appearing on a certain scanning line, and (b) shows the dual intensity signal which appears with taking the synchronism to the character-generating video signal. The high level portions of the character-generating video signal correspond to the brightened dot matrix elements which form the displayed characters. During the time periods when the dual intensity signal is kept to "H" state, the height of the high level portions of the character-generating video signal are held to a value designated as "H.sub.H ". During the time periods when the dual intensity signal is kept to "L" state, the height of the high levels portion of the character-generating video signal are held to a value designated as "H.sub.L " which is lower than "H.sub.H ". Therefore, characters generated from the dot matrix elements corresponding to the "H.sub.H " portions of the character generating video signal are accentuated by their display intensity on the CRT screen from those characters generated from the dot matrix elements corresponding to the "H.sub.L " portion of the output character generating video signal in accordance with the "H" or "L" state of the dual intensity signal.
Means for displaying the characters with dual intensity by the abovementioned method using the combination of the character-generating video signal and the dual intensity signal are known, and a conventional circuit which has widely been used heretofore is shown in FIG. 4. In this circuit, two digital switching elements (formed in ICs and called generally AND-gates) IC.sub.1 and IC.sub.2 are used. Input terminals A and B are connected in such a manner that the two input terminals of the IC.sub.1 are connected to terminal A, one another, and an input terminal of IC.sub.2. The other input terminal of the IC.sub.2 is connected to terminal B, as shown in the circuit diagram. And to the terminals A and B, an input character generating video signal and the dual intensity signal are applied, respectively. The output terminals of the AND-gates IC.sub.1 and IC.sub.2 are connected through a potentiometer type variable resistor VR.sub.1 to each other, and the character-generating signal is taken from its sliding tap terminal c. An example of the internal circuit configuration of the AND-gate IC's, IC.sub.1 and IC.sub.2 is shown in FIG. 5, wherein IN.sub.1 and IN.sub.2 represent the two input terminals of IC.sub.1 and IC.sub.2 or OUT represents the output terminal thereof. The detailed explanation of the operation of various parts of this circuit is omitted here, although the relevant function of this circuit to the operation of the external whole circuit is explained below. Only when both input terminals IN.sub.1 and IN.sub.2 are held in "H" state, a transistor TR.sub.5 turns on while a transistor TR.sub.6 turns off. A output terminal OUT is held in "H" state. When any one of remaining combinations of "H" and "L" other than the abovementioned occurs at the two input terminals IN.sub.1 and IN.sub.2, transistor TR.sub.5 turns off and the transistor TR.sub.6 turns on, whereby the output terminal OUT is held "L". Thus the AND-gate function by the IC is attained.
The principle of the operation of the conventional circuit shown in FIG. 4 is explained below. In this explanation, the effect of any internal resistance inside the AND-gates IC.sub.1 and IC.sub.2 is neglected.
First, when the dual intensity signal applied to the input terminal B is held to "H" state, as can be understood by the circuit connection between the input terminals A, B and each input terminal of two AND-gates IC.sub.1, IC.sub.2 shown in FIG. 4, IC.sub.1 and IC.sub.2 act in the same manner in accordance with the input character-generating video signal applied to the terminal A. That is, when the input character generating video signal is in "H" state, outputs of both AND-gates IC.sub.1 and IC.sub.2 are at "H" level; and when the input character-generating video signal is at "L" state, outputs of both IC.sub.1 and IC.sub.2 are in "L" level. Because of this simultaneously changing actions of two AND-gates IC.sub.1 and IC.sub.2, potentials at both ends of the potentiometer type resistor VR.sub.1 remain at the same value, therefore the potential at the sliding tap terminal c is also kept at the same values as at the both ends of VR.sub.1 regardless of the position of the sliding tap of the potentiometer VR.sub.1. Then, even if the sliding tap position of VR.sub.1 is adjusted, the signal at the terminal c varies only between fixed "H" and "L" levels in accordance with the input character-generating video signal applied to the terminal A.
Next, when the dual intensity signal applied to the terminal B is held in "L" state, since the output of IC.sub.2 is always kept to "L" level regardless of "H" or "L" state of the input character-generating video signal, potential at the lower end of VR.sub.1, i.e., of the end connected to IC.sub.2, is maintained at "L" level. On the other hand, potential at the IC.sub.1 -side end of VR.sub.1 varies between "H" and "L" levels in accordance with the input character-generating video signal. Then the value of the high level portion of the signal at the terminal c is a certain midway value between the "H" and "L" levels depending upon the position of the sliding tap of VR.sub.1.
As the result of the above-described operation of the AND-gates IC.sub.1, IC.sub.2 and the potentiometer type variable resistor VR.sub.1, the signal at the terminal c acts as the character-generating video signal in the dual intensity display mode shown by FIG. 3(a). Namely the high level of the signal at the terminal c during the period of time when the dual intensity signal is kept to "H" state, corresponds to the high level portion "H.sub.H " of FIG. 3(a); and the high level during the period of time when the dual intensity signal is kept to "L", corresponds to the lower high level portion "H.sub.L " of FIG. 3(a). That is, the high level "H.sub.H " is a fixed high level, being independent of the adjustment of VR.sub.1, the high level "H.sub.H " forming the accentuated characters with a fixed high display intensity on the CRT; while the lower high level "H.sub.L " is a variable high level, being adjustable to lower than "H.sub.H " with VR.sub.1, the lower high level "H.sub.L " forming the non-accentuated characters with a variable lower display intensity on a CRT screen. It should be noted that in the dual intensity display monitors such as described above, the display intensity of the accentuated characters is fixed, while that of the non-accentuated characters can be lowered than that of the accentuated characters by the adjustment.
In the circuit shown in FIG. 4, the character-generating video signal in the dual intensity display mode, obtained thus as an output at the terminal c, is fed through a base resistor R.sub.10 to a video amplifier stage, which comprises transistors TR.sub.10 and TR.sub.11 and amplifies up to a level sufficient to drive a display CRT 1. R.sub.11 is an emitter resistor, R.sub.12 and R.sub.13 are bias resistors, R.sub.14 is a collector resistor, and C.sub.10 is a base bias capacitor. The video amplifier stage is constructed so as to provide a flat wide band amplification from low frequencies up to high video frequencies by combining an emitter-grounded amplifier comprising the transistor TR.sub.10 with a base-grounded amplifier comprising the transistor TR.sub.11.
In the above explanation of the video amplifier circuit, which has been used heretofore in conventional dual intensity CRT display monitor, an idealized behavior of the circuit has been described. This idealization neglects the internal resistance of ICs which inevitably exists in real circuits. When considering this conventional circuit with the internal resistance, that is, in the real operation of the conventional video amplifier circuit with the dual intensity display function of FIG. 4, several undesirable points are present as will be mentioned later. On the other hand, the desirable points for the CRT display monitor with the dual intensity display mode are as follows:
(1) The display intensity of the non-accentuated characters can be adjusted fully between that of the accentuated characters and zero intensity. When the adjustment for the display intensity of the non-accentuated characters is set to the full intensity level, the non-accentuated characters, which are set by a certain programmed or data-controlled command, can be displayed in the same intensity as that of the accentuated characters: (the display intensity difference between the accentuated and non-accentuated characters is extinguished). On the other hand, when the adjustment for the display intensity of the non-accentuated characters is set to the zero intensity, only the non-accentuated characters can be erased completely from the CRT display.
(2) Even when the display intensity of the non-accentuated characters is changed by the adjustment operation, the display intensity of the accentuated characters is maintained at a fixed constant level, which is the highest level even when the display intensity of the non-accentuated characters is set to the highest level.
(3) If care is taken, it is possible to avoid the deterioration of the rise and fall characteristics of the character-generating video signal due to stray capacitance of cables. A cable connects a circuit board part, which contains the circuit of FIG. 4, to the potentiometer VR.sub.1, which is usually placed near the front panel of the apparatus separated from the circuit board part to facilitate control thereof.
When the internal resistance inevitably existing in circuit elements is taken into account, existing dual intensity CRT display monitor using the conventional circuit of FIG. 4 have the following undesirable points:
(1) The highest display intensity of the non-accentuated characters cannot be adjusted fully up to that of the accentuated characters. Since the IC.sub.2 -side end of the potentiometer VR.sub.1 is grounded, when the dual intensity signal is kept to "L" for displaying the non-accentuated characters, the current flowing through the potentiometer VR.sub.1 from its IC.sub.1 -side end to IC.sub.2 -side end becomes large. Namely, the potential at the IC.sub.1 -side end of the potentiometer VR.sub.1 cannot be kept to "H.sub.H " level due to the internal resistance of IC.sub.1, even when the input character generating video signal is in "H" state. Therefore, even when the sliding tap of the potentiometer VR.sub.1 is adjusted up to the IC.sub.1 -side end of VR.sub.1 for highest intensity display, the display intensity of the non-accentuated characters cannot be raised up to the intensity level of the accentuated characters. This inconvenience can be relieved to some extent but not eliminated by selecting a rather large resistance value for VR.sub.1.
(2) When the display intensity of the non-accentuated characters is changed by the adjustment of the potentiometer VR.sub.1, the display intensity of the accentuated character cannot be maintained at a fixed highest level. Adjustment of the sliding tap of VR.sub.1 for lowering the display intensity level of the non-accentuated characters causes an increase in the load resistance for IC.sub.1, thereby decreasing the potential at the IC.sub.1 -side end of the potentiometer VR.sub.1 depending upon the position of the sliding tap the potentiometer of VR.sub.1 due to the internal resistance of IC.sub.1. Therefore, even when the IC.sub.2 -side end of the potentiometer VR.sub.1 is held to "H" level by "H" state of the dual intensity signal for accentuated characters, the output signal at the terminal c decreases from its highest level "H.sub.H " depending upon the adjustment of the potentiometer VR.sub.1. Consequently the display intensity of the accentuated characters cannot be maintained at the high level which corresponds to the adjustment of the display intensity of the non-accentuated characters.
(3) Since cables connecting a circuit board part to the potentiometer VR.sub.1, which is usually placed near the front panel of the apparatus separated from the circuit board part to facilitate control thereof, are often lengthy, the stray capacitance of those cables inevitably becomes large. This causes the deterioration of the rise and fall characteristics of the input character-generating video signal. This deterioration is emphasized particularly when the resistance value of VR.sub.1 is selected to be a large value for promoting the greatest possible equality of the highest display intensity of the non-accentuated characters to the display intensity of the accentuated characters.